BAN: Make PlacementNew.h only include <new> if it exists

Otherwise it will define the functions itself. This allows compiling
toolchain as stdlibc++ depends on BAN.

BAN/include/BAN/PlacementNew.h

@@ -1,3 +1,10 @@
 #pragma once
 
-#include <new>
+#if __has_include(<new>)
+	#include <new>
+#else
+	#include <stddef.h>
+
+	inline void* operator new(size_t, void* addr) { return addr; }
+	inline void* operator new[](size_t, void* addr) { return addr; }
+#endif